question about Duration of stay??

This question has been answered many times, and equally many seem to be absolutely certain the right answer is A, and equally many seem to be absolutely certain it's B. Is there anyone here who knows the answer really for certain (having actually tested it themselves or asked the authorities)?

If the duration of stay on my visa is 30 days and I arrive in China on August 1, will I have to leave
A.) August 30 or earlier,
B.) August 31 or earlier?